Inhibitory effect of calcium-binding protein regucalcin on protein kinase C activity in rat renal cortex cytosol

The effect of regucalcin, a calcium-binding protein, on Ca2+-and phospholipid-dependent protein kinase (protein kinase C) activity in the cytosol of rat kidney cortex was investigated. With increasing concentrations of Ca2+, phosphatidylserine or dioctanoylglycerol in the reaction mixture, regucalcin (10[-8] M) caused a remarkable inhibition of protein kinase C activity. Regucalcin did not have a significant effect on protein kinase C activity in the presence of phosphatidylserine or dioctanoylglycerol without Ca2+ addition. Moreover, regucalcin significantly inhibited phorbol 12-myristate 13-acetate (PMA)-increased protein kinase C activity. Meanwhile, staurosporine (10[-9] M) caused a significant inhibition of protein kinase C activity. This inhibition was further enhanced by regucalcin addition. Regucalcin itself did not have protein kinase activity in either the presence or the absence of both Ca2+ and phospholipids. These results clearly indicate that regucalcin has an inhibitory effect on protein kinase C activity in the cytosol of rat kidney cortex. This inhibitory effect may be partly due to the regucalcin-induced Ca2+ binding.